Take a close look at phosphorus use

An expert answers questions about what phosphorus does for crops.

Dec 22, 2011

Take a close look at phosphorus use

Question: What does phosphorus do?

Jim Gray: Phosphorus is often called the “plant energizer.” It is vital to the plant growth process and essential for energy transfer and storage. P is the primary component of DNA in the plant. It promotes root growth and leaf expansion, which aids in early-season growth and health.

Question: How much P should I apply?

Gray: First, use a soil test to determine how much P and other nutrients are needed. This is also based on the crop. For 150-bushel corn, apply 60 pounds per acre of actual P. For 50-bushel soybeans, apply 45 pounds per acre actual P. For 3 tons of alfalfa, apply 45 pounds per acre actual P.

Question: How else can I maximize P?

Gray: As input costs continue to rise, the old days of just applying fertilizer “like Dad did” are over. Variable-rate application through the use of GPS could also correctly apply the nutrient where it is needed. It is possible some areas of a field will have higher levels of P and won’t even need additional application. By using variable-rate technology, it is possible to place the nutrients where there are lower levels.

Question: How should I apply P?

Gray: More and more farmers are using strip tillage or deep placement of fertilizers. Some are fertilizing for two crops at once; for example, the recommended levels needed for a soybean and corn crop are applied in one year in one application. Many times, P is applied as a starter fertilizer through the planter. This also helps incorporate the nutrient into the soil. In light of environmental issues dealing with P and runoff issues, strip tillage, deep placement and use as a starter through the planter may help reduce P runoff.

Question: What about dissolved P?

Gray: P will readily bind to the soil as long as there are available sites on the soil particle for the nutrient to bind to. If those sites are filled, it is possible to have too much P “load,” which can lead to leaching of dissolvable P. This is why it is advantageous to apply P near the root zone, so nutrients can be readily available to the plant and readily used; in addition, it lessens the risk of P ending up in streams, rivers and lakes from surface runoff.

Question: When should I apply P?

Gray: Fall is the best time to apply fertilizer. The dry soil works well for deep placement and incorporation with tillage. Fall is also the best time for broadcast applications. The fertilizer has time to break down with rain and get into the soil solution. Do not apply fertilizer to frozen ground.